1. Field of the Invention
The present invention relates to providing time information to a communications device.
2. Description of the Related Art
Positioning services have become very popular in the recent years. Positioning refers here to determining the location of a receiver device. The receiver device may be capable of determining its position based on signals it receives. The signals can be sent either from a specific positioning system or, for example, from a cellular communications system. Alternatively, the receiver device may act as a measurement device and send measurement results to a further unit, which then determines the location of the receiver device.
The most widely used positioning system is the Global Positioning System (GPS). GPS positioning is based on measuring relative time of arrival of signals sent simultaneously from GPS satellites. The locations of the GPS satellites at the time of sending the signal can be determined. It is possible to determine the location of the GPS receiver by determining the distance between GPS satellites and the GPS receiver using time of arrival measurement results together with the exact GPS time.
In theory, three time of arrival measurements would be enough to calculate the GPS receiver's position and also the velocity, if the exact GPS time is known to the GPS receiver. In practice, a GPS receiver has low-cost, low-accuracy local oscillator as a local clock. Therefore a fourth time of arrival measurement is typically needed to determine the difference between the local time and the GPS time. This means that for successfully locating a GPS receiver, it typically needs to receive signals simultaneously from at least four GPS satellites.
Each signal transmitted by a GPS positioning satellite carries a navigation message, which includes both data unique to the transmitting satellite and data common to all satellites. The navigation message contains time information, satellite clock correction data, ephemeris (precise orbital parameters), almanac (coarse orbital parameters), health data for all satellites, coefficients for the ionospheric delay model and coefficients to calculate the Universal Coordinated Time (UTC) from the GPS system time. The navigation message consists of 25 frames, and the frames are organized in such a way that a GPS receiver is able to obtain satellite-specific data (ephemeris) for exact position calculation within 30 seconds. This 30 seconds is the minimum time-to-first-fix (TTFF) of GPS in the general case. It takes 12.5 minutes to receive all the 25 frames completely.
As mentioned above, GPS positioning is dependent on obtaining accurate GPS time and navigation data and on performing distance measurements. For carrying out GPS positioning successfully, signals from three or four GPS satellites need to be received properly for demodulating navigation data needed for the distance measurements. GPS provides accurate positioning results especially in rural areas, where a GPS receiver can have a line-of-sight with the needed number GPS satellites. In urban areas, where buildings may cause attenuation of the GPS signals and reflections to the signal propagation paths, especially the reception of the navigation data may not be successful.
The distance measurements need to be performed at the GPS receiver, but the GPS time and navigation data may be provided to the GPS receiver also via another system. In Assisted GPS (AGPS), at least part of the GPS time and/or navigation data is provided as location assistance data to a GPS receiver by means of some other signals than by the GPS satellite signals. By providing navigation data and/or exact GPS time as location assistance information, the availability and the response time of GPS positioning can be enhanced. For example, the Time-To-First-Fix can be shortened. By obtaining location assistance information, a GPS receiver can perform distance measurements and optionally also calculate its position even when the GPS signals the GPS receiver receives are so weak that the navigation message cannot be properly demodulated. If the GPS time can be transferred to a GPS receiver with better accuracy than 2-3 seconds, it already helps in positioning. The best assistance is obtained with better than 10 microseconds accuracy.
A communications system, for example a cellular communications system, may be used for transmitting the location assistance information. The communications system may be equipped with a plurality of reference GPS receivers for obtaining and determining the location assistance information. Typically each reference GPS receiver is associated with a serving area. The location assistance information sent to GPS receivers within a serving area typically includes information relating to those GPS satellites, from which the reference GPS receiver of the respective serving area is able to successfully receive GPS signals. The GPS receivers, to which location assistance information is transmitted using a cellular telecommunications network, are typically integrated to a communication device capable of supporting communication via the communications system.
There are at least two alternatives for providing to a communication device information about a reference time for positioning. The first alternative is to tie the reference time of the positioning system to an event or events in the communications network. For example, the reference time may be tied to a start of a certain frame transmitted from a certain network element. For this purpose, time differences between base stations of a communication network typically need to be measured. A Location Measurement Unit is defined in the 3GPP (Third Generation Partnership Project), and it is responsible, for example, for measuring time differences between base stations and reporting them to a serving mobile location center. From the measurements the serving mobile location center defines the time correspondence between the reference time and GSM frames. Using this first alternative it is possible to deliver accurate reference time to a communications device. The accuracy may be better than 10 microseconds.
A second alternative is to estimate the current time of the positioning system in the communications system and send this information to the communications device. A GPS clock or receiver connected to the communication system may be used for determining current GPS time. This second alternative means, in practice, delivery of less accurate reference time. Providing reference time information without reference to external events in location assistance data is an example of this second alternative. When information about the reference time is sent to a communications device, there is a delay between the time instant when the reference time of the positioning system is determined and when the communications device receives the reference time information.
Especially if it is desired to provide a user-plane based positioning solution, the first alternative for providing reference time is not likely, as it would require a connection between a location measurement unit or a corresponding unit and a user-plane application.
To provide more accurate time information about the positioning system, it is possible to try to take into account the transfer delay. For example, for packet data transfer using IP (Internet Protocol) connections, a ping procedure may be used to measure the transfer delay. It is, however, possible that ping or similar procedures cannot be used due to data channel characteristics or standards support problems. As an example, ping assumes the existence of Internet Control Message Protocol (ICMP) in the communications device. The ICMP is part of the TCP/IP (Transfer Control Protocol/Internet Protocol) protocol suite. Ping therefore does not work if the communications device does not have a proper protocol stack. Moreover, depending on the settings and configurations of the network operator, the ICMP data packets that are used by ping, may not be able to reach the communications device. For example, the communications device does not have data connection subscription or some intermediate network element (such as a firewall or a proxy) blocks the IP packets carrying ICMP messages.
It is appreciated that although problems relating to providing accurate time information relating to GPS are discussed above, similar problems may arise in connection with delivering time information for other purposes.
Embodiments of the present invention aim to provide a feasible solution for providing time information accurately to a communications device.